The security threats of adware could range from man-in-the-middle attacks, which give the adware full access to your web traffic, to simple injection of scripts to create ads and popups.
Junk files accumulate through the normal use of your computer. Temporary files are created by application to help them run smoothly, or to back up your work. Files like this are no longer needed after they are used once, and don't get deleted automatically.
Malware like Found Malware are sometimes found embedded in programs supplied by official websites. It also possible such applications come bundled with useful programs.
As your computer gets older it collects unused programs which make it slower and slower. With the help of Removethisprogram.com, learning which files to keep or remove is easy.